This invention relates generally to the production of pellets used in the manufacture of glass and specifically to the process and apparatus utilized to dry pellets of agglomerated sand, limestone, and sodium carbonate or sodium hydroxide solutions utilized as preheated glass furnace feed.
Glass such as soda-lime glass is produced by reacting and melting sand, soda ash, and limestone or dolomite and other glass batch ingredients in a furnace to form a homogeneous melt. The use of sodium carbonate in the glass batch introduces dust particles which are entrained in furnace gases. On contact with lining materials in the furnace, sodium carbonate dust particles accelerate the attack of the refractory materials resulting in increased maintenance costs. One method known to suppress Na.sub.2 CO.sub.3 dust formation is to add water to the glass batch prior to its being fed to the furnace. This water must be evaporated in the high temperature atmosphere of the furnace and results in a curtailment of furnace melting capacity and an increase in fuel consumption per unit of glass produced.
An improvement is obtained by the substitution of solutions of sodium hydroxide for water and a portion of the sodium carbonate used. U.S. Pat. No. 3,149,983 issued Sept. 22, 1964, to L. Maris et al describes the use of caustic soda with soda ash in the production of glass making batches containing sand. Glass batches produced by this method have a tendency to cake and result in handling difficulties.
South African patent application No. 69-6971 by C. A. Sumner teaches the preparation of agglomerated glass batch ingredients in a rotary drum having rods to develop a falling curtain of particles onto which a caustic soda solution is sprayed. Similarly, British Pat. No. 1,282,868 issued July 26, 1972, to F. G. West-Oram teaches the production of a glass batch in pellet form from sand, limestone, and caustic soda in a rotary dryer with flights. The pellets formed are heated to remove water and to accelerate the reaction of the caustic soda with the sand.
Agglomerates prepared by the processes of South African Application No. 69-6971 and British Pat. No. 1,282,868 as well as agglomerates produced in disk-type pelletizing apparatus are formed in sequential pelletization and drying stages where caustic in the interior of the pellet is not completely carbonated. Such pellets are, therefore, hygroscopic and permit segregation of the soluble Na.sub.2 O component during drying. These properties result in handling and storage problems and lead to non-homogeneous compositions of the molten glass.
The function of a drying step in the pelletization process is generally to remove water supplied during the pelletizing process from the individual pellets. The drying process generally strengthens the pellets enabling them to withstand the rigors of handling and transport, as well as providing a better melting characteristic. However, the drying of pellets or the heating of solid agglomerates prior to feeding to a melting furnace, such as that utilized in glass making, presents a number of difficulties. Exposing moist pellets to higher temperatures during an uncontrolled drying process can result in crumbling, smearing, or exploding of the pellets. This break-down of the pellets may be due to excessive strain induced by high thermal gradients within the agglomerated material or to excessive internal pressures such as occurs with super-heated steam where the rate of water volatilization exceeds the rate of vapor diffusion to the internal pores of the individual pellets. Disintegration of the pellets normally occurs because the heat transfer rates to the interior of the pellets are in excess of the water vapor diffusion rates to the exterior. This normally results in the explosion of the pellets.
An additional problem to maintaining pellet integrity occurs when pellets are formed where crystalline sodium carbonate monohydrate (Na.sub.2 CO.sub.3.H.sub.2 O) serves as a bonding agent. Crystalline sodium carbonate monohydrate has the apparent tendency to weaken as a bonding agent in a glass pellet batch at temperatures above a specific temperature range because of the release of the water of hydration. This weakens the monohydrate bonding within the individual pellets. Above this predetermined range the bond reforms as the pellet dries due to the evaporation of the water of hydration. At temperatures above this sensitive temperature range, anhydrous carbonate serves as a bonding agent to bond the pellets so that no substantial amount of pellet degradation occurs. However, within this sensitive temperature range the pellets suffer substantial degradation if they are subjected to continuous or appreciable impingement against dryer flighting and each other or to continuous mechanical movement during the final drying and preheating process.
In contrast, the very slow heating and drying of pellets results in very slow rates of water volatilization wherein the evaporation is normally restricted to the outer surface of the pellets. This allows the soluble components combined with the water to migrate to the surface of the pellet, thereby producing a heterogeneous composition from the surface inwardly. This latter is especially unacceptable for pellets used in the manufacture of glass since the more easily fusible soda-rich layer on the outside flows away from the pellet during fusion in the glass furnace and leaves a more refractory silica core which melts more slowly. Such a process causes defects in the quality of glass produced because of the differences in density and viscosity.
The foregoing problems are solved in the design of the process and apparatus of the present invention by providing a static bed dryer that minimizes pellet agitation and carefully controls pellet bed and drying and heating gas temperature differentials. The process and apparatus is utilizable with rotary pelletizing apparatus or any other appropriate apparatus which produces agglomerated glass batch ingredients in pelletized form for dehydration and preheating prior to the pellets being fed to a melting furnace.